In hydrocarbon reforming, a hydrocarbon feedstock and steam are reacted catalytically in a reformer furnace to form a synthesis gas comprising hydrogen and carbon monoxide. The reforming furnace is a critical component of hydrogen production facilities and plants which use synthesis gas to produce methanol and ammonia, and can account for almost half of the operating costs and energy expenditures of such installations.
A typical reforming furnace comprises a fired radiant section, a transition section, and a convection section. Tubes filled with a reforming catalyst, i.e. reformer tubes, are disposed in the radiant section. The reforming catalyst is typically nickel on an alumina support. A hydrocarbon feedstock and steam are fed through and reformed in the reformer tubes. Combustion of a fuel in the radiant section produces a hot flue gas which heats the reformer tubes and provides the thermal energy necessary for the endothermic reforming reaction. The transition section receives hot flue gas from the radiant section and passes it to the convection section. One or more heating coils disposed in the convection section may be used for different preheat purposes, including preheating the hydrocarbon feedstock and steam before that feed stream is reformed in the reformer tubes. Flue gas from the transition section heats the convection section coils.
The overall efficiency of a reforming furnace is affected by the absorbed heat duty of the reformer tubes. In general, greater reformer tube heat duties require increased temperatures and firing rates in the radiant section. Operating the radiant section in this way requires increased maintenance and shortens the reformer tubes useful life. Increasing the heat duty provided by the radiant section can also lead to coke formation in the reformer tubes.
Maximizing the temperature of the hydrocarbon feedstock and steam at the inlet of the radiant section catalyst tubes (the “preheat temperature”) improves reformer efficiency. At higher preheat temperatures, the reforming reaction is initiated closer to the inlet of the catalyst tubes, which improves the efficiency of the reforming reaction.
FIG. 1 illustrates a conventional reforming process in which a mixed feed 11 comprising a hydrocarbon feedstock and steam is preheated in heat exchange tube 42 in the convection section 40 of the reformer 10. Preheated mixed feed 13 then flows through reformer tubes 12 located in radiant section 20 to form a synthesis gas product stream 15. Fuel is combusted in the radiant section 20 of the reformer, external to the reformer tubes, to provide heat for the reforming reaction. A combustion product gas mixture is withdrawn from the radiant section and passed to the transition section 30 and subsequently passed to the convection section 40. Synthesis gas product stream 15 is collected at the outlet end of the reformer tubes and is supplied to a customer after additional purification. A conventional reforming process such as that illustrated in FIG. 1 can only achieve a preheat temperature of around 500° C. to around 600° C. due to the risk of carbon formation from the heavy hydrocarbons present in the feedstock.
A reduction in radiant section heat load can be achieved by reforming a portion of the higher hydrocarbons in the hydrocarbon feedstock in a prereformer prior to feeding the mixed feed to the reformer tubes. Partially reforming the hydrocarbon feedstock and steam prior to introducing the mixture into the primary reformer tubes is known in the art as “prereforming.” This approach is illustrated in FIG. 2 and in U.S. Pat. No. 5,264,202. Referring to FIG. 2, a mixed feed 11 of a hydrocarbon feedstock and steam is preheated in heat exchange tube 42 in the convection section 40 of reformer 10 to form preheated mixed feed 13 which is fed to prereformer 50. Prereformer effluent stream 24 may be heated to a reheat temperature of around 680° C. in convection section 40 prior to being fed to the inlet of reformer tubes 12 in radiant section 20 of reformer 10. Synthesis gas product stream 15 is collected at the outlet end of the reformer tubes.
FIG. 3 illustrates a variant of the process of FIG. 2 in which prereformer tubes 17 are positioned within the reformer transition section 30. In the process illustrated in FIG. 3, a mixed feed 11 comprising a hydrocarbon feedstock and steam is fed to prereformer tubes 17. Effluent from the prereformer tubes is passed to heat exchange tubes positioned in the convection section 40 of the reformer 10 and heated to a reheat temperature. The prereformed mixed feed is then fed at the reheat temperature to reformer tubes 12. Prereforming within a convection/transition section prereformer is also described in U.S. Pat. No. 6,818,028. In general, prereforming proves useful for reforming a natural gas feed as a means to reduce steam generation or primary reformer duty and in instances where the conversion from one feedstock to another is required.
The preheat and prereforming process designs described above prove unsuitable for expanding the production capacity of an existing reformer above around 25% of existing capacity because they are constrained by the energy available in the flue gas, the radiant section firing duty, convection section space limitations, and the overall plant heat balance.
Heat exchange tubes in the reformer convection section which carry effluent from the prereformer tubes are typically exposed to reformer section radiant heat and a varying radiant section flue gas temperature. This makes it difficult to regulate the reheat temperature of the prereformed gas.
Installation of a prereformer in the transition section of an existing reformer is very expensive. The reformer must be taken off-line, thereby disrupting the output of all associated facilities. Limited space in the convection section often precludes installation of an adequately-sized prereformer. Further, the convection section coil design may interfere with the positioning of prereformer tubes in the convection section.
These problems can be compounded by the fact that heat in the flue gas coming from the reformer radiant section may be inadequate to heat both the convection section coils and the prereformer tubes. In such cases, installing a prereformer in the convection section could disrupt the energy balance of the reformer and all associated plants. An “associated plant” is any facility that receives or uses hydrogen-containing product gas produced by a reformer.
Major changes to all of the convection coils downstream of the prereformer might also be required.
Accordingly, the need exists for a cost-effective process which expands the production capacity of an existing reformer through preheat and prereforming without disrupting the output or energy balance of the either the reformer or any associated plant.